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The long term impact of the Seli One shipwreck on the Table Bay beaches

Seifart, Christian (2012-12)

Thesis (MScEng)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: On the 9th September 2009, the 178 m Panamanian bulk carrier, the Seli One, ran aground off the coast of
Blouberg in Table Bay, South Africa. Due to failed salvage attempts, the vessel has remained stranded
approximately 500 m off the Blouberg beachfront. Since the vessel ran aground, a gradual change in the
Blouberg beach shape in the lee of the wreck has been observed. The local coastline, which has traditionally
been fairly uniform, has assumed a curved shape, with significant sediment accretion being observed in the
wave shadow of the wreck. Initially, the Seli One wreck remained intact. However, during a storm on the 4th
September 2011, the vessel split up into three separate pieces.
The impact that the wreck is having on the local wave, current and sediment transport dynamics remains
undefined. This lack of knowledge results in significant risks, relating to shoreline stability and beach amenity.
The objective is this study was therefore the determination of the long-term impact of the Seli One shipwreck
on the Blouberg beachfront.
A review of existing literature has indicated that no empirical relationships are available which could be used
to calculate the impact of a shipwreck on nearby coastal processes. Numerous methods are available which
can be used to determine the net longshore transport rates, but these cannot be used to quantify the impacts
of shipwrecks on the local sediment transport regime. Numerical models were therefore used to determine
the impact of the Seli One shipwreck.
Through the analysis of simulation results, it was concluded that, as expected, the shipwreck has resulted in a
significant reduction in the net longshore sediment transport rate in her lee, resulting in sediment deposition
in this area. It was further concluded that the vessel does not result in the complete blockage of longshore
sediment transport, and that sediment is able to periodically pass through the lee of the vessel.
The simulated beach salient on the 3rd July 2011 was compared to results of a beach survey, performed on the
same date specifically for this study. The simulated accretion of approximately 27 m in the lee of the
shipwreck agrees well with the measured salient. It has been shown that approximately 75% of the salient
accretion occurred within the first two months of the vessel’s arrival. Furthermore, shoreline erosion on the
northern side of the salient resulting directly from the shipwreck has been shown to be approximately 15 m.
This too occurs relatively rapidly, within approximately two months of the vessels arrival.
Following the initial impact of the wreck in its intact configuration, the long-term potential impact of the vessel
in its broken-up configuration was determined. This included the assumption that the vessel does not undergo
any additional breaking-up, and remains in its three-piece configuration indefinitely. This has shown that the
salient width resulting from the shipwreck is reduced to approximately 20 m, compared to the initial 27 m.
However, shoreline erosion on the northern side of the wreck has increased from approximately 15 m initially
to approximately 18 m in the long-term, which is caused by the continuous sedimentation between the vessel
and the beach.
A two-dimensional coupled wave, current and sediment transport model has been developed and has shown
that the wave shelter resulting from the Seli One results in the formation of a submerged salient between the
vessel and the shoreline.
It was found that shipwrecks have the potential of significantly altering local longshore sediment transport
characteristics in general. Depending on local conditions, this may pose serious risks, both in terms of
jeopardizing local seaside infrastructure, as well as creating dangerous swimming conditions.
Considering the impact that a shipwreck can have on local shoreline changes, with special regard to the rate at
which these shoreline changes can occur, it is recommended that the results obtained from the current study
be used to estimate the impact of potential future shipwreck scenarios in Table Bay.